1. Field of the Invention
The present invention relates to an image processing apparatus for correcting the color of an image output from a printer, an image processing method therefor, and a program for generating image processing parameters.
2. Description of the Related Art
With the improved performance of electrophotographic apparatuses in recent years, some electrophotographic apparatuses have achieved an image quality equivalent to that of a printing machine. However, there remains a problem that such electrophotographic apparatuses have a larger color variation amount than that of a printing machine because of the instability specific to the electrophotographic process. Therefore, conventional electrophotographic apparatuses employ various calibration techniques.
To correct primary colors, a conventional electrophotographic apparatus employs a calibration technique for generating a look up table (LUT) for correcting one-dimensional (1D) gradation corresponding to the cyan, magenta, yellow, and black toners. The LUT refers to a table indicating output data corresponding to input data delimited at specific intervals. The LUT enables expressing nonlinear characteristics that cannot be represented by arithmetic formulas. A color produced by using each of the C, M, Y, and K toners is referred to as “monochromatic color”. Performing “monochromatic” calibration enables correcting the monochromatic color reproduction characteristics, such as a maximum density and a gradation.
In recent years, Japanese Patent Application Laid-Open No. 2011-254350 discusses a technique for performing “multi-color” calibration by using a four-dimensional (4D) LUT. “Multi-color” means a color produced by a plurality of color toners. For example, the red, green, and blue colors are produced by using two out of the C, M, and Y colors. The gray color is produced by using the C, M, and Y colors. Particularly with electrophotography, expressing “multi-color” by using a plurality of color toners often produces a nonlinear difference even if the monochromatic gradation characteristics are corrected by using a one-dimensional LUT. In this case, executing multi-color calibration enables correcting the color reproduction characteristics of a multi-color expressed in combination (superposition) of a plurality of color toners.
The following describes a flow of calibration including “multi-color” calibration. First of all, to execute “monochromatic” calibration, a patch image is printed on a recording medium, such as paper, by using monochromatic chart data. This patch image having a single density and a predetermined area is used for measurement. When a plurality of patch images having different colors is generated and printed on a recording medium, these path images are collectively referred to as a pattern image. The recording medium, such as paper, on which the pattern image is printed is read by using a scanner or a sensor to read the patch images. Data obtained by reading the patch images is compared with a preset target value to generate a one-dimensional (1D) LUT for correcting the difference from the target value. Then, to execute “multi-color” calibration, patch images are printed on a recording medium by using multi-color chart data reflecting the 1D LUT generated previously, and then the patch images are read by using a scanner or a sensor. Data obtained by reading the patch images is compared with a preset target value to generate a four-dimensional (4D) LUT for correcting the difference from the target value.
As described above, high-precision correction is feasible by correcting through “multi-color” calibration the multi-color characteristics which cannot be corrected only through “monochromatic” calibration.
A result of calibration is affected if the density and color of toner applied to paper is change by the type of the sheet (paper type). Therefore, it is important to associate the paper type with the target value preset for each paper type. Japanese Patent Application Laid-Open No. 2007-272112 discusses a technique for selecting paper at the time of execution of each calibration, selecting internal parameters suitable for the selected sheet, and executing calibration.
With the conventional technique, paper to be used is separately optimized at the time of execution of each calibration. When a plurality of calibrations for different correction targets is executed, such as monochromatic calibration and multi-color calibration, it is necessary to select paper types to be used before execution of each calibration, or to select a sheet feed stage storing paper belonging to the selected paper type. The user needs to make setting in this way before execution of each calibration. As a result, workloads on user operations increase causing a problem that user's intervention cannot be reduced in calibration processing.
Although the target value used for correction in monochromatic calibration differs for each paper type, calibration may be performed, in some cases, so that the density ratio in halftone and maximum density value becomes constant for each paper type. In this case, since the applied toner amount corresponding to an output signal differs for each paper type, the applied toner amount required to output a certain color image differs for each paper type.
Specifically, after execution of monochromatic calibration by using different paper, a different color is output since a correction table differs for each paper type.
Under this condition, a target value for multi-color calibration is registered. To register a target value at the time of execution of multi-color calibration, a toner image actually printed on paper is measured, and the result of the measurement is registered as a target value.
Since this target value for multi-color calibration is registered after execution of monochromatic calibration, the target value differs for each type of paper which has been used at the time of execution of monochromatic calibration.
For example, when an image processing apparatus supporting paper types A and B performs monochromatic calibration by using paper of the paper type A, the apparatus outputs an image by using a correction table A.
In multi-color calibration executed in this case, the apparatus uses a selected one of two different target values (a target value A1 registered by using paper of the paper type A and a target value B1 registered by using paper of the paper type B).
On the other hand, when the same image processing apparatus performs monochromatic calibration by using paper of the paper type B, the apparatus outputs an image by using a correction table B which is different from the correction table A. Therefore, since color correction is performed by using a different correction table from the one previously used, the apparatus also needs to change the target value.
Therefore, in multi-color calibration executed in this case, the apparatus uses a selected one of two different target values (a target value A2 registered by using paper of the paper type A and a target value B2 registered by using paper of the paper type B).
Specifically, a plurality of target values for multi-color calibrations is required for each paper type of paper that has been used for monochromatic calibration.
As described above, if two different paper types are usable at the time of execution of monochromatic calibration and at the time of execution of multi-color calibration, there are two different target values for the paper types for multi-color calibrations corresponding to each of the two different types of paper for monochromatic calibration. Specifically, a total of four different target values are required for multi-color calibration.
Registering a plurality of target values for multi-color calibrations for each paper type used at the time of execution of monochromatic calibration takes time and effort in this way, resulting in complicated processing.
Therefore, it is desired to conform a paper type of paper to be used at the time of execution of monochromatic calibration to a paper type of paper to be used at the time of execution of multi-color calibration.